Many oilfield operations include the use of a fluid being circulated through or otherwise introduced into the borehole. These fluids may include drilling fluids that are circulated during the drilling of the well, completion or production fluids that may be circulated during or after drilling during various completion operations, and fracturing (“fracking”) fluids which may be used after drilling in order to stimulate the well to increase production from a hydrocarbon reservoir.
After a well is drilled into a subterranean geological formation that contains oil, natural gas, and water, efforts are made to maximize the production of the oil and/or gas. To increase the permeability and flow of the oil and/or gas to the surface, the drilled wells are often subjected to well stimulation. Well stimulation generally refers to several post drilling processes used to clean the well bore, enlarge channels, and increase pore space in the interval to be injected thus making it possible for fluids to move more readily into the formation.
A well stimulation process may generally include pumping engineered fluids at high pressure and rate into the subterranean geological formation. The fluid (usually water with some specialty high viscosity fluid additives) exceeds the rock strength and opens a fracture in the formation, which can extend out into the geological formation for as much as several hundred feet. Certain commonly used fracturing treatments generally comprise a carrier fluid (usually water or brine) and a polymer, which is also commonly referred to as a friction reducer. Many well stimulation fluids will further comprise a proppant. Other compositions used as fracturing fluids include water with additives, viscoelastic surfactant gels, gelled oils, crosslinkers, oxygen scavengers, and the like.
Aqueous fluids are also used at various other stages in the production of oil and/or natural gas. For example aqueous fluids may be used during any stage of drilling, production, fracturing, or completion of a wellbore. Similarly, aqueous fluids may be used during refining processes or as a component of treatment fluids for oilfield equipment such as pipes, lines, manifolds, or other conduits, tanks and storage vessels, and transport containers. Water may also be stored at an oilfield location, e.g. in a reservoir or pond, for use in oilfield processes (such as injection into a wellbore). This water may be stored above ground in a reservoir or pond or may be in a subterranean reservoir that can be accessed as necessary.
Water may be present in any of these processes, either alone or as a component of a wellbore or oilfield fluid. However, the water should be suitably free from undesirable microbes. If the water in these oilfield applications contains threshold levels of microbes, such as bacteria, fungus, etc., these can grow and proliferate on any surface with which the fluid is in contact or downhole. If left untreated, microbes and microbial biofilms (slimes) can cause deterioration of equipment, loss of efficiency in equipment, promotion and acceleration of corrosion on metal surfaces, or increased down time. For example, bacteria pose a risk to a well and production equipment because they can become attached to equipment and corrode metal, significantly weakening it or even to the point of failure. Bacteria can also grow and reproduce in the formation or oilfield apparatus and certain bacteria can produce H2S (hydrogen sulfide) resulting in a soured well. A soured well is can reduce the value of the asset by as much as 50%.
In addition to water used in oilfield fluids, a further source of microbes can be solid components incorporated into oilfield fluids. A common example may be proppant particles incorporated into wellbore stimulation fluids. These solids may have microbes deposited on their surface or incorporated in the solid particles. These microbes may then be released when the solids are incorporated into oilfield fluids and may inoculate the aqueous fluid with a microbial population.
Biocides and antimicrobials may be used to control microbial growth in the water. As used herein, “control” is defined to include both inhibition and removal.
Bacterial levels may be measured in oilfield fluids using a serial dilution method in which a sample is sequentially diluted tenfold in six progressive samples to give logarithmic dilutions 1×, 10×, 100×, 1,000×, 10,000×, 100,000×. These samples are placed under standard conditions conducive to bacterial growth and observations of bacterial growth are made over a period of time. The results may be provided in the industry in the form of a “log” number which represents the highest dilution in which bacterial growth was observed under the test conditions. The results from these standard tests can take days or even weeks to return.
The concept of measure, monitor, and control is an approach to the selection, application and post-application monitoring of oilfield operations, e.g. hydraulic fracking, production, etc. In order to tailor an antimicrobial treatment appropriately, monitoring of the microbial content of the water in the specific target process or fluid is advised to allow selection of appropriate microbial control parameters.